Fish Sensory Systems

Fish may be cold-blooded, but they're not insensitive

The future of fisheries management may involve the sensibilities of sensitive fish. Restoration projects might include "odorant bouquets." Bubble curtains might become tactile barriers that keep invasive carp in check. Maybe your future in "managing" fish will include an eye-catching new lure. Here's what scientists are learning about fish senses:

Odor and Scent Trails

It was only about 50 years ago that Arthur Hasler, a leading figure in 20th century freshwater ecology, revealed that adult salmonids locate their home streams for spawning by following distinctive scent trails remembered from their youth. Since then, research laboratories like the one Peter Sorensen, professor in the Department of Fisheries, Wildlife and Conservation Biology, operates at the University of Minnesota have shown that odor trails leading trout and salmon "home" are paved with bile acids, amino acids, and possibly even components of the home stream itself, like calcium.

"We've shown that Kamloops are so sensitive to the smell of certain bile acids that they can detect them at concentrations of less than a thimble-full in a billion gallons of water," said Sorensen.

Sorensen and others in his field think that bile acids might eventually be used to lure trout and salmon back to locations where they are now threatened. Conversely, sea lamprey pheromones could become a means for controlling invasive populations in the Great Lakes. "One pound of lamprey pheromones dribbled slowly down Niagara Falls could have sea lamprey responding for a month," said Sorensen. "One of the great powers of olfaction is the response can be extremely specific. Finding the right odor is difficult, but when you have it, it's extremely potent and easy to add to water."

When researchers plug the nares (nasal passages) of species like salmon and lamprey, the fish become disorientated, lose their ability to spawn successfully, and struggle to detect prey or predators. Multiple studies have shown that waterborne contaminants like herbicides, pesticides and metals can effectively plug a fish's nares.

"Salmonid research has reached a strange place," said Sorensen. "Spawning runs have all but vanished in streams on the West Coast. It could be that contaminants related to development are scrambling the fishes' homing abilities but testing this idea in the field would be difficult since there are other variables that could be damaging salmon populations."

Whiff for whiff, salmonids have an olfactory prowess that is greater than your own but Sorensen considers them a "pretty average type of fish" when it comes to an ability to detect odors. He puts eels, lamprey (a creature with 1/3 of its brain devoted to olfaction), and catfish among the world's top fish sniffers. He also puts emphasis on the fact that pheromones, as they are strictly defined, are not what anglers will use to catch fish. In the realms of science, a pheromone is a chemical (or a suite of chemicals) emitted by an individual that triggers a specific reaction in a member of the same species. A more accurate descriptor for the odors sold for scenting lures is a class of semiochemicals (information chemicals) that one fish species emits and a different species (typically a predator) can interpret.

Brian Wisenden, professor in the Biosciences Department at Minnesota State University Moorhead, explains that some manufacturers of bait attractants say their products contain pheromones because the word has evolved in lay-terms to mean "any chemical that contains information." Wisenden and his colleagues have published research indicating that strike rate is seven times higher on a bait scented with extract from the skin of fathead minnows over baits scented with non-native fish or plain water.

Sight

"Generally, trout species have a well-developed sense of sight," said Tom Hrabik, associate professor of biology at the University of Minnesota Duluth. Hrabik and his graduate students are interested in how the eyesight of deepwater Lake Superior fish works in relation to the wavelengths of light available at different depths. To do this they converted a near-freezing room into a near-freezing laboratory with darkened lights and shrouded aquaria. "The room is lit with a weird blue-green color, like you were about 20 meters down in the lake," said Hrabik.

The young lake trout in the first phase of the investigations were exceptionally adept at seeing prey in very low light levels in the blue-green wavelength spectrum, especially if the prey was moving. As light passes through water, most of the visible spectrum is absorbed within 10 meters. Even in the best of conditions, only the short blue spectrum wavelengths can make it to depths of 150 meters.

Like your eyes, fish eyes contain both rods and cones. Their eyes are replete with the three chemicals that allow humans to see in a seven-color spectrum, plus a fourth chemical. The fourth chemical, common to most predatory fish, permits them to experience the ultraviolet range.

Another fish-eye feature is "eye shine." Eye shine helps fish like walleye and deep-sea species to see well despite their dimly lit world. Reflected light bounces off a mirror-like layer near the back of the eye allowing light to pass through the eye twice. Raccoons and other mammals that favor the night have the same layer in their eyes.

Vincent Marinaro made a fish's view accessible in 1976 when his popular book, In The Ring of the Rise, hit the shelves. Marinaro used high-speed photography to visually parse what trout see. He showed, and others concur, that trout see through the "trout's window" (also known as "Snell's window," referencing the Dutch mathematician who gave us the law of refraction). The window is a cone shaped viewshed extending up from the trout's eye at an increasing diameter. The diameter of the window is a little more than twice the depth of the trout. The reflectivity of the surface of the water acts like a mirror from underneath, making it difficult for a fish to see beyond the "trout's window" during the day.

Fisheye camera lenses capture the world as a fish might see it: thorough an ultra-wide hemispherical view. Compared to the flat lenses of humans and other animals, a fish's rigid, spherical lens is adapted for seeing close-up in water, a medium where light bends more than it does in air. When a fish needs to see at a distance, special muscles pull the entire lens back toward the retina.

Sight is typically the first sense a trout uses when deciding whether or not to strike at a fly ... even at night. With eyes on either side of its head, a fish can perceive two images at the same time. When an image is viewed by only one eye, its two-dimensionality prevents the fish from judging distances. When both eyes focus on the image, the visual arcs overlap creating a narrow band where the fish achieves binocular vision and can judge distance.

Hearing

Do not underestimate a trout's ability to hear you. Sound travels approximately five times faster and, depending on the tone, much farther in water than in air. Although not well, sound vibrations can also pass from air to water to a fish's internal hearing apparatus.

William Mowbray in Sounds of the Sea, and Grant Gilmore, Jr. in Oceanic Biological Sounds: Mating Calls From the Sea bore witness to the ability of fish to make and respond to sound. An exceptional example is the toadfish. Male oyster toadfish emit frog-like croaks to attract females during spawning. The decibel level approximates that of a car horn heard from 5 meters away, which is so loud that their shenanigans detonated acoustic mines in the ocean during World War II, forcing the U.S. Navy to withdraw the mines from the U.S. seaboard. You can hear the sound at: www.wikipedia.org/wiki/Oyster_toadfish.

Although slandered by the media as one of "the ugliest and laziest fish known to inhabit the waters of the northeast," oyster toadfish have made significant sacrifices on behalf of humanity and because of their ears. Our vestibular (or balance) system relies on fluid-filled canals in the ear. The toadfish's vestibular system is similar, but their nerve layout is less intricate. Research involving toadfishes' ears is improving therapies for motion sickness, Meniere's disease and other balance disorders, and some hearing impairments. These hardy fish even orbited Earth with astronauts as illustrious as former Sen. John Glenn for studies of how balance adjusts to changes in gravity.

Touch

Toadfish have been integral to understanding a sensitivity that no human possesses - the ability to detect subtle changes in water pressure through a lateral line system.

Sometimes referred to as the "sense of distant touch," lateral lines convert subtle changes in water pressure into electrical pulses similar to the way our inner ear responds to sound waves. Running lengthwise down each side of the body and over the head, these pressure-sensing organs help their owners avoid collisions, participate in schooling behavior, orient to water currents, elude predators, and detect prey.

Lateral lines are composed of neuromasts (hair cells surrounded by a protruding jelly-like cup) that usually lie at the bottom of a visible pit or groove. These hair cells - the same sensory cells found in all vertebrate ears - convert mechanical energy into electrical energy when moved. Presumably, auditory and lateral line pathways evolved in close association since they share many features.

Allen Mensinger, associate professor with the University of Minnesota Duluth Department of Biology, and his colleagues discovered that a toadfish's lateral line fires messages to the brain when a prey fish approaches within half a body length. The wave of water produced when a prey fans its fins is like a whisper ... one that can be sensed by a toadfish when it is generated a body length away and understood at half that distance.

"Species like walleye and steelhead are more difficult to work with because they are less hardy," said Mensinger. "But as we continue to refine our electrode and telemetry equipment, we're anticipating opportunities to include fish from Lake Superior's waters in our research. Having the ability to observe pulses through the nervous system in free-swimming fish opens new avenues for understanding fish behavior such as home stream migration and schooling."

Mensinger reports that studies of fluid dynamics are advancing our understanding of lateral lines. Researchers are finding that fish produce donut-shaped waves as they swim and that predators likely track prey through the resulting vortices. He also said that new studies have shown that man-made fish ladders create a type of turbulence that is offensive to a fish's sense of touch. This insight is inspiring engineers to design with fish sensitivity to touch in mind.

Like you, fish have nerve endings on the surface of their body that allow them to feel their surroundings. Catfish, with their scale-free skin and barbly chins, are particularly adept at feeling their way through the water.

Taste and More

Along with tactile nerve endings on the surfaces of their bodies, fins, and mouths, most fish species have the ability to taste through their skin and lips. For example, sharks (and even trout) have been observed rubbing their bodies across potential prey presumably to determine if it was truly food. Catfish, along with having exceptional tactile sensitivity, have been called "swimming tongues." Their skin is coated with taste buds, which helps them find food in the murky bottom waters of lakes and rivers.

Sorensen explains that there are three chemical sensors in fish: a common chemical sense, olfactory nerves, and taste buds. A trout's common chemical sense allows it to respond to irritants and chemicals in the water. Olfaction allows it to process complex phenomena whereas taste produces a reflexive snapping behavior. "Trout respond to about three or four compounds with a snapping reflex," Sorensen said. "If they are hunting they likely first see the prey, then they smell it, and then if their taste sensors are activated by the compounds, they can't help but snap at the potential prey item."

Electroreception - the ability to detect electric fields - is a sense that trout probably don't possess, but sturgeon and other fish respond to electrical currents. The receptors of the electrical sense are modified hair cells of the lateral line system.